Switch to stay ON until power removed (24V)

Question

In my bathroom, I would like to install some 24v LED tape, some in a warm-white and some in a cool-white, that I can switch between at will. IE: I will turn on the circuit from the wall-mounted dimmer outside the bathroom and then select either the warm or cool LEDs via another switch. I then want the ability to switch from one to the other at will. This, I realise will be easy - Install a switch after the 24v transformer to alternate between the two.

However, I am curious whether a switch exists that will default to the warm-white circuit. So, for example, when I turn on the circuit at the wall the warm-white LEDs will be powered by default. I can then switch to the cool white however, when I turn off the power to the whole circuit at the wall the switch will automatically switch back to the default warm-white circuit so that next time I turn on the lights they are on the warm-white. Maybe some kind of self-powered relay? Electro-magnetic switch?

To add more complexity this will be after the 24v transformer that will be dimmable by PWM. If I have to I suppose it could be done on the mains supply with one transformer for each of the circuits.

So, to summarise: Default to circuit 'A' at power ON, switch to circuit 'B' after switch press, revert to circuit 'A' after power OFF.

Thank you in advance to anyone who may be able to help!

Answer 1

How about this?:

^{simulate this circuit – Schematic created using CircuitLab}

Your post mentions transformer but it must be more than that to power the LEDs. I'm assuming it's a PWM supply of some sort. If that's the case each pulse will be full voltage and we can 'steal' some of that to charge a capacitor to make our power supply. C1 is charged up by the PWM pulses via D3 (to prevent discharge) and R1 (to limit current draw). You'll need to calculate the values of these components to suit your relay coil.

Pressing S1 will energise the relay and latch itself on while turning on the cool LEDs. When the PWM source is switched off C1 will discharge through the relay coil and reset the circuit.

If the external switch is a regular dimmer switch then this circuit is unlikely to work as the LED power supply is, more than likely, a simple transformer and rectifier.

Answer 2

The following circuit should do everything you asked for, and here's how it works:

U1 is a "D" type flip-flop, and when the 5 volt supply is turned on, R2 and C2 integrate the supply's rising edge and present it as a momentary signal starting at zero volts, to U1-S, which forces U1Q high, to +5 volts, and U1Qbar and U1D low, to 0 volts (ground).

U1Q going high turns on Q1 which energizes K1, causing the LED supply to be connected to the warm-white LED string, turning that LED string ON.

S1 is a normally-open momentary switch and, in conjunction with R1C1 (a switch debouncer), is used to send clock pulses to U1>

A "D" type flip-flop works by transferring whatever's on its D input to its Q output as soon as it sees a high-going edge on its clock input, so since D is low, Q will go low when S1 is pressed.

That'll cause the transistor to turn OFF which, which will de-energize K1, disconnecting the LED supply from the warm-white string and and connecting it to the cool white string through the relay's normally-closed contact.

D is now high because it's connected to Qbar, so if S1 is pressed again, the transistor will will turn ON again and...

Thus, on power-up, the default state will be warm-white and, after that, every time the switch is pressed, the LED strings will change states.

Answer 3

The circuit below makes no assumptions about whether a separate switch is used to turn off the LED PWM (meaning one could run the +5 supply off the same switch), or it is part of the dimmer circuit itself, as it uses a missing pulse detector to determine when the PWM signal has quit rather than using a loss of power. It also doesn't try to "harvest" power from the PWM source, since at very low light levels (a few % on), I don't think there would be enough to pull in a relay coil.

The PWM is fed into a voltage divider that cuts the 24V down to 5V (signal conditioning or other circuitry be be needed, since I don't know what that signal really looks like).

The output of the retriggerable one-shot 74LS123 stays high as long as there is a PWM pulse train. The keeps the \$\small \overline{\text{CL}}\$ input of the 74LS74 flip-flop (being used as a simple latch) high, so the flip-flop is not being cleared.

The period of the one-shot just needs to be longer than 1/frequency of the PWM; I set it arbitrarily to 500 ms since I have no idea what it looks like. The formula for the time is T (in ms) = 0.33 * R (in K) * C (in µF). In this case, T = 0.33 * 150K * 10 µF = 495 ms.

If the pushbutton is pushed, this places a low on the \$\small\overline{\text{PR}}\$ input of the 74LS74, setting it and turning on the relay, switching the output from the warm LEDs to the cool ones. The relay will not turn off until the one-shot times out meaning the PWM train of pulses has ended.

To see a larger version, right-click and click on View Image.